As demand for smaller, more powerful electronic devices grows, semiconductor manufacturers are constantly attempting to reduce the size and cost of semiconductor packaging. Smaller packages equate with higher semiconductor mounting densities and higher mounting densities allow for more capable yet more compact devices.
With conventional packaging methods, a semiconductor die or “chip” is encapsulated in a ceramic or plastic package having a number of electrical leads extending therefrom. The leads permit electrical connection between external components and the circuits on the die. Although these packages have proven reliable, they are generally many times larger than the actual die. In addition, the configuration of these packages typically yields only a limited number of leads. For these reasons, conventional packaging techniques are not particularly adaptable to miniaturization.
As a result, recent developments in semiconductor packaging have led to more efficient chip packages. For example, “pin grid arrays” or PGAs utilize a series of pins extending from an entire face of the die (or other electronic device). The pins are electrically connected by traces or bond wires to the electrical circuits on the die. Electrical interconnection is produced by inserting the array of pins into mating sockets on a receiving component. While PGAs provide increased electrical interconnection density, the pins forming the PGA are fragile and easily bent. In addition, the PGA is relatively expensive to produce.
Similar to the PGA is the “ball grid array” or BGA. Instead of pins, the BGA has an array of solder bumps or balls attached to one side of the semiconductor device. The array of solder bumps is adapted to mate with a receiving component wherein the package may be subsequently heated to partially liquefy or “reflow” the bumps, thus forming electrical connections at the discreet ball locations. Like the PGA, the BGA improves conductor density by utilizing the entire device face for electrical interconnection. The present invention is directed primarily to BGAs and the remainder of this discussion will focus on the same.
While BGAs have proven reliable, there are known problems. Primarily, current procedures used to attach the solder balls are expensive or otherwise not conducive to high volume, low cost manufacturing. For instance, one method of solder ball attachment involves an automated robotic arm that picks up individual solder balls, applies a flux thereto, and precisely places the balls at the correct locations on the device. Heat is then applied to reflow the solder balls and thus secure the balls to the device. While effective, this method requires highly specialized and expensive machinery. Furthermore, the machine must be reconfigured for each BGA pattern.
Another method for locating and attaching solder balls to form a BGA uses a fixture having a stencil or silk screen overlay with a series of apertures corresponding to the desired ball locations or bond pads on the device. After the stencil is brought into registration with the electronic device, flux is applied through the apertures to coat the bond pads. A solder paste is then squeegeed over the stencil, filling the apertures. Due to the relative viscosities of the flux and paste, the paste adheres temporarily to the flux-covered bond pads. To avoid adhesion to the stencil, it is typically made from a non-wettable material. The stencil is then removed and the assembly is heated to reflow the solder paste wherein the paste forms a solder ball at each location. Alternatively, the stencil may remain until after reflow.
Like the robotic machines, solder paste techniques are not without problems. One particular problem with solder paste is that ball placement density or “pitch” is limited by the viscosity of the solder paste. In particular, the paste can flow outwardly from its intended bond pad once the stencil is removed. In addition, fine pitch stencils can accommodate only limited solder volume, restricting the size of the resulting ball. Furthermore, the process of applying the solder paste to the stencil is time consuming and difficult to completely and efficiently automate.
A variation of the paste method involves placing solder balls instead of solder paste into the stencil. Here, the electronic device and the stencil may be placed into an alignment fixture. A plurality of solder balls may then be placed over the stencil. By agitating the fixture, the solder balls “drop into” the apertures and are temporarily retained in the layer of flux on each bond pad. Like the solder paste techniques described above, the balls may be reflowed to attach to the bond pads. Unfortunately, this method of attachment is also time consuming and difficult to automate.
Yet another method of solder ball placement uses a flexible carrier or preform having embedded solder balls therein. With the assistance of an alignment fixture, the preform is brought into edge registration with the electronic device. The preform may then be reflowed to secure the embedded solder balls to the device. While effective in overcoming some of the stencil problems, removal of the carrier requires an aqueous rinse or manual removal procedures. For this and other various reasons, the preform attachment method is used primarily in low volume applications.
Another method of solder ball placement is described in U.S. Pat. No. 5,861,323 issued to Hayes on Jan. 19, 1999. Hayes describes a insulating membrane holding an array of solder balls wherein the solder balls protrude through both sides of the membrane. The membrane is then precisely placed between two components and the solder reflowed. While effective, Hayes requires that the membrane and both electronic components be held in precise alignment during reflow. Furthermore, the membrane remains in place between the two components after solder reflow.
Still yet another method of solder ball placement is shown in U.S. Pat. No. 5,219,117 issued to Lin on Jun. 15, 1993. Lin discloses a transfer substrate having a plurality of recesses which align with bond pads on a semiconductor device. While overcoming some of the problems with the other attachment techniques, the transfer substrate does not positively retain the solder balls after insertion into the recesses. Accordingly, the substrate must be carefully handled prior to reflow to prevent dislodging of the balls from the substrate. Furthermore, the method described by Lin does not secure the transfer substrate to the semiconductor device prior to reflow. As such, the substrate and device must be held in precise alignment by external means during reflow.
Accordingly, what is needed is a system of locating and securing solder balls in a manner that is conducive to high volume BGA production. What is further needed is a system which can economically and reliably position the solder balls on a high density (fine pitch) semiconductor device and secure the balls thereto until subsequent reflow.